Preparation and use of methionylmethionine as feed additive for fish and crustaceans

ABSTRACT

An animal feed mixture containing DL-methionyl-DL-methionine and salts thereof for animals kept in aquacultures is provided. Methods for preparing DL-methionyl-DL-methionine of formula (I) 
     
       
         
         
             
             
         
       
     
     and methods to fractionate the diasteriomeric forms obtained are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is continuation of U.S. application Ser. No. 13/616,533filed Sep. 14, 2012, pending, which is a divisional of U.S. 12/580,283filed Oct. 16, 2009, U.S. Pat. No. 8,968,817, and claims the benefit ofU.S. 61/117,361 filed Nov. 24, 2008 and DE 10 2008 042 932.5 filed Oct.17, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to novel chemical syntheses ofmethionylmethionine, the dipeptide of methionine, and the specific usethereof as feed additive alone or mixed with methionine for fish andcrustacean nutrition.

2. Description of the Related Art

Essential amino acids (EAA) such as methionine, lysine or threonine arevery important constituents as feed additives in animal nutrition andplay a significant part in the commercial rearing of productive animalssuch as, for example, chickens, pigs and ruminants. Supplementation ofnatural protein sources such as, for example, soybeans, corn and wheatwith EAAs makes it possible on the one hand for the animals to growfaster, or for milk production to be higher in high-output dairy cows,but on the other hand for the utilization of the feed to be moreefficient. This represents a very great commercial advantage. Themarkets for feed additives are of great industrial and commercialimportance. In addition, they are high-growth markets, attributable notleast to the increasing importance of countries such as, for example,China and India.

L-Methionine ((S)-2-amino-4-methylthiobutyric acid) represents the firstlimiting amino acid for many species such as chickens, ducks, turkeysand also for many fish and shellfish species and therefore plays a verysignificant part in animal nutrition and as feed additive (Rosenberg etal., J. Agr. Food Chem. 1957, 5, 694-700 and Lovell, T. R., J. Anim.Sci. 1991, 69, 4193-4200). However, in the classical chemical synthesis,methionine results as racemate, a 50:50 mixture of D- and L-methionine.This racemic DL-methionine can, however, be employed directly as feedadditive because there is in some species under in vivo conditions atransformation mechanism which converts the unnatural D enantiomer ofmethionine into the natural L enantiomer. This entails firstly theD-methionine being deaminated with the aid of a nonspecific D-oxidase toα-ketomethionine, and subsequently being further transformed with anL-transaminase into L-methionine (Baker, D. H. in “Amino acids in farmanimal nutrition”, D'Mello, J. P. F. (ed.), Wallingford (UK), CABInternational, 1994, 37-61). The available amount of L-methionine in thebody is increased thereby and can then be available to the animal forgrowth. The enzymatic transformation of D- to L-methionine has beendetected in chickens, pigs and cows, but especially also in carnivorousand omnivorous fish and also in shrimps and prawns. Thus, for example,Sveier et al. (Aquacult. Nutr. 2001, 7 (3), 169-181) and Kim et al.(Aquaculture 1992, 101 (1-2), 95-103) were able to show that thetransformation of D- into L-methionine is possible in carnivorousAtlantic salmon and rainbow trout. Robinson et al. (J. Nutr. 1978, 108(12), 1932-1936) and Schwarz et al. (Aquaculture 1998, 161, 121-129)were able to show the same for omnivorous fish species such as, forexample, catfish and carp. In addition, Forster and Dominy (J. WorldAquacult. Soc. 2006, 37 (4), 474-480) were able to show in feedingexperiments on omnivorous shrimps of the species Litopenaeus vannameithat DL-methionine has the same activity as L-methionine.

The world production in 2007 of crystalline DL-methionine and racemic,liquid methionine hydroxy analog (MHA,rac-2-hydroxy-4-(methylthio)butanoic acid (HMB)) and solid calcium MHAwas more than 700,000 t, which was successfully employed directly asfeed additive for monogastric animals such as, for example, poultry andpigs. Owing to the rapid commercial development of fish and crustaceanfarming in highly industrialized aquacultures an optimal, economical andefficient methionine supplementation option has become increasinglyimportant precisely in this area in recent years (Food and AgricultureOrganization of the United Nation (FAO) Fisheries Department “State ofWorld Aquaculture 2006”, 2006, Rome, International Food Policy ResearchInstitute (IFPRI) “Fish 2020: Supply and Demand in Changing Markets”,2003, Washington, D.C.). However, in contrast to chickens and pigs,various problems occur on use of methionine, MHA or Ca-MHA as feedadditive for certain fish and crustacean varieties. Thus, Rumsey andKetola (J. Fish. Res. Bd. Can. 1975, 32, 422-426) report that the use ofsoybean meal in conjunction with singly supplemented crystalline aminoacids did not lead to any increase in growth of rainbow trout. Murai etal. (Bull. Japan. Soc. Sci. Fish. 1984, 50, (11), 1957) were able toshow that daily feeding of fish diets with high rates of supplementedcrystalline amino acids in carp led to more than 40% of the free aminoacids being excreted via the gills and kidneys. Because of the rapidabsorption of supplemented amino acids shortly after feed intake, thereis a very rapid rise in the amino acid concentration in the fish's bloodplasma (fast response). However, at this time, the other amino acidsfrom the natural protein sources such as, for example, soybean meal arenot yet present in the plasma, possibly leading to asynchronicity of theconcurrent availability of all the important amino acids. As a resultthereof, part of the highly concentrated amino acids is rapidly excretedor rapidly metabolized in the body, and is used for example as pureenergy source. As a result, there is only a slight or no increase ingrowth, upon use of crystalline amino acids as feed additives (Aoe etal., Bull. Jap. Carp Soc. Sci. Fish. 1970, 36, 407-413). Supplementationof crystalline amino acids may lead to further problems in crustaceans.The slow feeding behavior of certain crustaceans such as, for example,shrimps of the species Litopenaeus Vannamei results, owing to the longresidence time of the feed under water, in the supplemented,water-soluble amino acids being dissolved out (leaching), leading toeutrophication of the water and not to an increase in growth of theanimals (Alam et al., Aquaculture 2005, 248, 13-16).

Efficiently supplying fish and crustaceans kept in aquacultures thusrequires, for certain species and applications, a specific methionineproduct form, such as, for example, an appropriately chemically orphysically protected methionine. The aim of this is on the one hand thatthe product remains sufficiently stable in the aqueous environmentduring feeding and is not dissolved out of the feed. On the other handthat the methionine product eventually taken in by the animal can beutilized optimally and with high efficiency in the animal body.

Many efforts have been made in the past to develop suitable feedadditives, particularly based on methionine, for fish and crustaceans.Thus, for example, WO8906497 describes the use of di- and tripeptides asfeed additive for fish and crustaceans. The intention of this is topromote the growth of the animals. However, the di- and tripeptidespreferably employed in this case were from nonessential and thereforealso nonlimiting amino acids such as, for example, glycine, alanine andserine. The only methionine-containing dipeptides described areDL-alanyl-DL-methionine and DL-methionyl-DL-glycine. However, this meansthat effectively only 50% of active substance (mol/mol) are present inthe dipeptide, and this must be categorized as very disadvantageous fromthe aspect of economics. WO02088667 describes the enantioselectivesynthesis and use of oligomers of MHA and amino acids such as, forexample, methionine as feed additives, inter alia also for fish andcrustaceans. It is said to be possible to achieve faster growth thereby.The described oligomers are assembled by an enzyme-catalyzed reactionand exhibit a very broad distribution of the chain lengths of theindividual oligomers. This makes the process unselective, costly andelaborate in the procedure and purification. Dabrowski et al. describesin US20030099689 the use of synthetic peptides as feed additives forpromoting the growth of aquatic animals. In this case, the proportion ofthe peptides in the complete feed formulation may be 6-50% by weight.The synthetic peptides preferably consist of essential and limitingamino acids. However, the synthesis of such synthesized oligo- andpolypeptides is very elaborate, costly and difficult to convert to theindustrial scale. In addition, the effectiveness of polypeptides of asingle amino acid is disputed, because these are often converted onlyvery slowly or not at all under physiological conditions into free aminoacids. Thus, for example, Baker et al. (J. Nutr. 1982, 112, 1130-1132)describes the lack of biological value of poly-L-methionine in chickensbecause of the absolute insolubility in water, since absorption by thebody is impossible.

Besides the use of novel chemical methionine derivatives such as, forexample, methionine-containing peptides and oligomers, there has alsobeen investigation of various physical protection possibilities such as,for example, coatings and the incorporation of an amino acid in aprotective matrix. Thus, for example, Alam et al. (Aquacult. Nutr. 2004,10, 309-316 and Aquaculture 2005, 248, 13-19) were able to show thatcoated methionine and lysine has, in contrast to uncoated, a verypositive influence on the growth of young kuruma shrimps. Although useof a specific coating was able to suppress the leaching of methionineand lysine out of the feed pellet, there are some serious disadvantages.The preparation or the coating of methionine usually represents atechnically complicated and elaborate process and is therefore costly.In addition, the surface coating of the methionine after coating iseasily damaged by mechanical stress and abrasion during feed processing,possibly leading to a diminution or complete loss of the physicalprotection. An additional factor is that the content of methionine isreduced, and thus often becomes uneconomic, by a coating or use of amatrix substance.

Besides the inventive novel use of DL-methionyl-DL-methionine as feedadditive with low leaching characteristics from feed pellets andextrudates, and an optimal supply of methionine to the body throughslow-release cleavage of methionylmethionine, it has also been possibleto develop novel processes for preparing methionylmethionine which havemany advantages over the preparation variants described in theliterature. Most of the dipeptide syntheses disclosed in the literatureuse costly protective groups such as, for example,Boc-(tert-butoxycarbonyl) or Z-(benzyloxycarbonyl) protective groups,which have to be attached to the appropriate amino acid before theactual dipeptide synthesis, and subsequently eliminated again. Inaddition, activation of the amino acids to be coupled is usuallynecessary. Thus, methionylmethionine can be prepared by couplingN-Boc-methionine with the methyl ester of methionine usingdicyclohexylcarbodiimide (DCC). The great disadvantages of thispreparation process are the use of costly protective groups, a veryelaborate synthesis and costly coupling reagents which cannot berecycled, such as, for example, DCC. Another alternative for theindustrial synthesis of methionylmethionine is described in DE2261926.3,6-Bis[2-methylthio)ethyl]-2,5-piperazinedione(methioninediketopiperazine, DKP) is formed in the first stage byheating the isopropyl ester of methionine and is then hydrolyzed tomethionylmethionine. Merely satisfactory yields of 62-65% were possiblefor the hydrolysis step in this case. In addition, the use of methionineisopropyl ester as starting material is too costly and thereforeuneconomic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagrammatic representation of the enzymatic cleavage ofthe methionylmethionine diastereomer mixtures DD/LL-I, DL/LD-I andDD/LL/DL/LD-I.

FIG. 2 shows a diagrammatic representation of the enzymatic cleavage ofthe four methionylmethionine diastereomers DD-I, LL-I, DL-I and LD-Iwith different rates of cleavage.

FIG. 3 shows a diagrammatic representation of the enzymatic liberationof methionine (D- and L-Met together) from the four methionylmethioninediastereomers DD-I, LL-I, DL-I and LD-I.

FIG. 4 shows the biotransformation of D-methionine to L-methionine withan enzyme cocktail from common carp.

FIG. 5 shows the leaching characteristics of methionylmethioninediastereomer mixtures DD/LL-I, DL/LD-I and LL/DD/LD/DL-I compared withmethionine, MHA and MHA-Ca.

FIG. 6 shows the in vitro digestion of four differentmethionylmethionine diastereomers LL-I, LD-I, DL-I and DD-I withdigestive enzymes of the common carp.

FIG. 7 shows the in vitro digestion of various methionylmethioninediastereomer mixtures LL/DD-I, DL/LD-I and LL/DD/LD/DL-I with digestiveenzymes of the common carp.

FIG. 8 shows the in vitro digestion of four differentmethionylmethionine diastereomers LL-I, LD-I, DL-I and DD-I withdigestive enzymes of the rainbow trout.

FIG. 9 shows the in vitro digestion of the methionylmethioninediastereomer mixtures LL/DD-I, DL/LD-I and LL/DD/LD/DL-I with digestiveenzymes of the rainbow trout.

FIG. 10 shows the in vitro digestion of four differentmethionylmethionine diastereomers LL-I, LD-I, DL-I and DD-I withdigestive enzymes of the whiteleg shrimps.

FIG. 11 shows the in vitro digestion of various methionylmethioninediastereomer mixtures LL/DD-I, DL/LD-I and LL/DD/LD/DL-I with digestiveenzymes of the whiteleg shrimps.

DETAILED DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a feedstuff or afeed additive for animal nutrition based on a novel methioninesubstitute which can be employed alone or as mixture with methionineespecially in the sector of industrial fish and crustacean farming inaquacultures. This and other objects have been achieved by the presentinvention, the first embodiment of which includes an animal feed mixturecomprising a nutrient selected from the group consisting ofDL-methionyl-DL-methionine, a salt thereof and a mixture ofDL-methionyl-DL-methionine and a salt thereof.

A second object of the present invention is to provide a simple andcost-effective chemical synthesis of this novel methionine substitute.This objective has also been achieved by the present invention, afurther embodiment of which includes a process for preparingDL-methionyl-DL-methionine of formula (I), comprising:

reacting a urea derivative of formula II to obtainDL-methionyl-DL-methionine;

wherein the urea derivative of formula II is one derivative selectedfrom the group consisting of IIa, IIb, IIc, IId, IIe, IIf and IIg, and

R¹ and R² in the urea derivatives IIa, IIb, IIc, IId, IIe, IIf and IIgare defined as follows:

IIa: R¹=COOH, R²=NHCONH₂ IIb: R¹=CONH₂, R²=NHCONH₂ IIc: R¹=CONH₂, R²=NH₂IId: R¹—R²=—CONHCONH— IIe: R¹=CN, R²=OH IIf: R¹=CN, R²=NH₂ IIg: R¹==O,R²=H.

In the light of the disadvantages of conventional synthesis methods, anobject of the present invention is to provide a chemically protectedmethionine product for various omnivorous, herbivorous and carnivorousfish and crustacean species which live in salt or fresh water. It isintended in particular that this product show low solubilitycharacteristics (leaching) from the complete feed pellet or extrudate inwater and possess a slow-release mechanism, i.e. a slow and continuousrelease of free methionine under physiological conditions. In addition,the novel methionine product of the present invention may be employedadvantageously as a mixture with DL-methionine.

In a further embodiment, the present invention provides a methioninesubstitute as feedstuff or a feed additive which has very highbiological value and which is easy to handle and store and has goodstability under the usual conditions of compound feed processing,especially pelleting and extrusion.

In another embodiment, the present invention provides fish andcrustaceans with a further efficient methionine source, besidescrystalline DL-methionine, which source exhibits if possible thedisadvantages of the known products to only a reduced extent or not atall.

In a further embodiment, the present invention provides a novel,flexible synthesis route for methionylmethionine(DL-methionyl-DL-methionine) in which the typical precursors andbyproducts from the industrial DL-methionine production process may beused as starting material. In a still further embodiment, the presentinvention provides a process for separating the pairs of diastereomersDD/LL- and DL/LD-methionylmethionine, so that an optimal and efficientuse of only one pair of diastereomers (DL/LL-I or DL/LD-I) may bepossible for specific applications.

Within the context of the present invention, all ranges below includeexplicitly all subvalues between the upper and lower limits.

In a preferred embodiment of the present invention,DL-methionyl-DL-methionine and salts thereof are provided as a feedadditive in feed mixtures for animals kept in aquacultures. The feedmixture may comprise from 0.01 to 5% by weight, preferably comprises0.02 to 3.0% by weight and most preferably comprises from 0.05 to 0.5%by weight of DL-methionyl-DL-methionine.

The use of DL-methionyl-DL-methionine is particularly advantageous inthis connection because the compound shows excellent leachingcharacteristics because of the low solubility of the mixture ofDD/LL/DL/LD-methionylmethionine and of the pair of diastereomersDL/LD-methionylmethionine (0.4 g/l).

The compound further shows good pelleting and extrusion stability duringfeed production. DL-Methionyl-DL-methionine is stable in mixtures withconventional components and feedstuffs such as, for example, cereals(e.g. corn, wheat, triticale, barley, millet, inter alia), vegetable oranimal protein sources (e.g. soybeans and oilseed rape and the productsof the processing thereof, legumes (e.g. peas, beans, lupins, etc.),fish meal, inter alia) and in combination with supplemented essentialamino acids, proteins, peptides, carbohydrates, vitamins, minerals, fatsand oils.

It is a further advantage of the present invention that, one mole ofwater is saved per mole of methionylmethionine compared withDL-methionine owing to the high active substance content ofmethionylmethionine per kg of substance.

In a preferred embodiment, the feed mixture may comprise proteins andcarbohydrates, preferably based on fish meal, soybean meal or corn meal,and may be supplemented with essential amino acids, proteins, peptides,vitamins, minerals, carbohydrates, fats and oils.

It is particularly preferred for the DL-methionyl-DL-methionine to bepresent in the feed mixture solely as DD/LL/LD/DL mixture, as DL/LD orDD/LL mixture, preferably in each case additionally mixed withDL-methionine, preferably with a DL-methionine content of from 0.01 to20% by weight, most preferably of from 0.5 to 15% by weight andparticularly preferably of from 1 to 10% by weight.

In a particularly preferred embodiment of the present invention,DL-methionyl-DL-methionine may be a DL/LD-methionylmethionine pair ofenantiomers.

In a preferred method of feeding animals according to the presentinvention, the animals kept in aquacultures are fresh and salt waterfish and crustaceans selected from the group consisting of carp, trout,salmon, catfish, perch, flatfish, sturgeon, tuna, eels, bream, cod,shrimps, krill and prawns, very preferably for silver carp(Hypophthalmichthys molitrix), grass carp (Ctenopharyngodon idella),common carp (Cyprinus carpio) and bighead carp (Aristichthys nobilis),carassius (Carassius carassius), catla (Catla Catla), Roho labeo (Labeorohita), Pacific and Atlantic salmon (Salmon salar and Oncorhynchuskisutch), rainbow trout (Oncorhynchus mykiss), American catfish(Ictalurus punctatus), African catfish (Clarias gariepinus), pangasius(Pangasius bocourti and Pangasius hypothalamus), Nile tilapia(Oreochromis niloticus), milkfish (Chanos), cobia (Rachycentroncanadum), whiteleg shrimp (Litopenaeus vannamei), black tiger shrimp(Penaeus monodon) and giant river prawn (Macrobrachium rosenbergii).

According to the invention, DL-methionyl-DL-methionine (I)(methionylmethionine or Met-Met for short) or its alkali metal andalkaline earth metal salts such as, for example, the slightly solublecalcium or zinc salt may be used as addition in feed mixtures asDD/LL/DL/LD, DD/LL or DL/LD diastereomer mixture, alone or mixed withDL-methionine, preferably for fish and crustaceans:

Four different stereoisomers (diastereomers) exist of the dipeptideDL-methionyl-DL-methionine (I), DD-, LL, DL- and LD-I, of which onlyL-methionyl-L-methionine (LL-I) is natural, all the other threedipeptides L-methionyl-D-methionine (LD-I), D-methionyl-L-methionine(DL-I) and D-methionyl-D-methionine (DD-I) being unnatural (see scheme1).

In this connection, DD-I and LL-I are related to one another as imageand mirror image, i.e. they are enantiomers and thus have the samephysical properties. The same applies to the DL-I and LD-I pair.

The two pairs DD/LL-I and DL/LD-I are by contrast diastereomers of oneanother, i.e. they have different physical data. Thus, for example, theDD/LL-I pair of diastereomers has a solubility of 21.0 g/l in water atroom temperature, whereas the solubility of the DL/LD-I pair ofdiastereomers is 0.4 g/l.

Besides providing novel synthetic methods for preparingmethionylmethionine, the present invention, in a further embodiment,provides a method employing DL-methionyl-DL-methionine as feedstuff asDD/LL/DL/LD, DD/LL or DL/LD diastereomer mixture as growth promoter foromnivorous, carnivorous and herbivorous fish and crustaceans inaquacultures. According to an embodiment of the present invention,DL-methionyl-DL-methionine (I) may be cleaved under physiologicalconditions enzymatically by fish and crustaceans to free D- andL-methionine (scheme 2) (see also examples 22 to 24). For this purpose,the corresponding digestive enzymes have been isolated from carp(omnivore), trout (carnivore) and whiteleg shrimp (omnivore) and reactedwith DL-methionyl-DL-methionine in optimized in vitro experiments underphysiologically comparable conditions. The particular feature accordingto the invention of the cleavage of DL-methionyl-DL-methionine (I) isthat all four possible diastereomers, both the natural LL-I, and thethree unnatural diastereomers DD-, DL- and LD-I may be cleaved underphysiological conditions. This may apply both to the use of the completemixture of all diastereomers (DD/LL/DL/LD-I), and in each case to thetwo pairs of diastereomers DD/LL-I and DL/LD-I (see FIG. 1).

However, the cleavage of the individual diastereomers ofmethionylmethionine takes place at different rates. This is illustratedby the diagrammatic representation of the enzymatic cleavage of theindividual diastereomers of methionylmethionine with digestive enzymesof fish and crustaceans in FIG. 2. However, the delayed cleavage meansthat the liberation of D- and L-methionine is likewise delayed (see FIG.3). This has the great advantage that there can be no fast-responseabsorption of free D- or L-methionine in the digestive tract and thus noconcentration peak of free methionine in the blood plasma either.

The advantage of using methionylmethionine as feed additive andmethionine source according to the present invention may thus be that D-or L-methionine is liberated in the body over the whole digestion periodand thus proceeds synchronously with the release of other amino acidsderived from natural protein sources (slow-release mechanism) (see FIG.3). This special effect results in the simultaneous availability of allthe important and essential amino acids in an ideal ratio in the bloodplasma being ensured, as is absolutely necessary for an optimal growthof the body.

In the enzymatic cleavage of the DL-methionyl-DL-methionine dipeptide(I), the unnatural D-methionine is also liberated in addition to thenatural L-methionine (see scheme 2). The former may be enzymaticallytransaminated both by carnivorous, omnivorous and herbivorous salt andfresh water fish and crustaceans to give natural L-methionine. This isshown for the example of carp in example 25. With the aid of an enzymecocktail of digestive and liver enzymes from carp, D-methionine may betransformed into L-methionine under physiologically correspondingconditions (see FIG. 4). An optimal supply of natural L-methionine tothe body may thus be ensured by use of DL-methionyl-DL-methionine (I).

The pelleting and extrusion experiments with various mixtures ofDL-methionyl-DL-methionine (I) and natural protein and carbohydratesources such as, for example, fish, corn and soybean meal, and mixedwith other essential amino acids, proteins, peptides, vitamins,minerals, fats and oils, show that DL-methionyl-DL-methionine (I) isabsolutely stable during and after the production process and nodegradation or decomposition whatsoever occurs (see example 26).

In order to investigate the leaching characteristics of thediastereomers of methionylmethionine (I) from compound feed pelletsunder water, the time-dependence of the dissolving out ofmethionylmethionine was measured (see example 26). For comparison, theleaching characteristics of DL-methionine, MHA and calcium-MHA (MHA-Ca)were investigated under identical conditions. This study shows that boththe complete mixture of all the diastereomers (DD/LL/DL/LD-I) and thepairs of diastereomers DD/LL-I and DL/LD-I show distinctly less leachingthan DL-methionine, MHA and calcium-MHA (MHA-Ca) (see FIG. 5). Much lessmethionylmethionine is thus dissolved out of the feed pellets over timethan with all other methionine derivatives. Particularly low leachingrates are shown by the DL/LD-I pair of diastereomers, a maximum of only5% of which was dissolved out of the feed pellets even after a residencetime of 200 min (see FIG. 5).

A further preferred embodiment of the present invention provides aprocess for preparing DL-methionyl-DL-methionine of formula (I)

by reacting a urea derivative of formula II

wherein the radicals R¹ and R² in the urea derivatives IIa, IIb, IIc,IId, IIe, IIf and IIg are defined as follows:

IIa: R¹=COOH, R²=NHCONH₂ IIb: R¹=CONH₂, R²=NHCONH₂ IIc: R¹=CONH₂, R²=NH₂IId: R¹—R²=—CONHCONH— IIe: R¹=CN, R²=OH IIf: R¹=CN, R²=NH₂ IIg: R¹==O,R²=H

to give DL-methionyl-DL-methionine (I).

In one embodiment of the process of the invention it is moreoverpreferred for methioninehydantoin (IId) to be the starting material orto be formed as intermediate product. In this process,DL-methionyl-DL-methionine is synthesized directly frommethioninehydantoin and includes methods G, H, and J shown in scheme 3.

In a preferred embodiment of this method, a solution comprisingmethioninehydantoin and water may be reacted with methionine under basicconditions. It is further preferred for the pH of the solutioncomprising the urea derivative to be adjusted to a range from 8 to 14,preferably to from 9 to 13.5 and most preferably from 10 to 13.

In a further preferred embodiment, the reaction takes place at atemperature of from 50 to 200° C., preferably at a temperature of from80 to 170° C. and particularly preferably at a temperature of from 130to 160° C.

It is further preferred for the reaction to be carried out underpressure, preferably under a pressure of from 3 to 20 bar, morepreferably 4 to 18 bar and particularly preferably under a pressure offrom 6 to 15 bar.

In a further preferred embodiment of the process of the presentinvention, a solution comprising methioninehydantoin and water may bepreviously formed from one or more of the compounds IIa, IIb, IIc, IId,IIe, IIf and IIg.

In another preferred embodiment of the process, methioninehydantoin maybe obtained by reacting the compound IIe or IIf with anitrogen-containing base, NH₄HCO₃, (NH₄)₂CO₃, NH₄OH/CO₂ mixture orcarbamate salts. Reaction of the compound IIe may be preferably carriedout at a temperature of from 0° C. to 150° C., more preferably 0° C. to100° C. and particularly preferably from 10° C. to 70° C.

In still another preferred embodiment of the process, themethioninehydantoin is obtained by reacting the compound IIf with CO₂.In this embodiment, it is preferred that the reaction to take place inthe presence of a base, preferably selected from the group comprisingKHCO₃, K₂CO₃, tertiary amines or salts thereof, alkali metal andalkaline earth metal bases.

In an additional further preferred embodiment of the process,methioninehydantoin is obtained by reacting the compound IIg with acyanide ion source and a base selected from the group includingnitrogen-containing bases, ammonium salts in the presence of CO₂,NH₄HCO₃, (NH₄)₂CO₃, NH₄OH/CO₂ mixture and carbamate salts. The reactionin this case takes place at a temperature of preferably −20° C. to 150°C., preferably −10° C. to 100° C. and particularly preferably from 0° C.to 70° C.

An alternative embodiment of the process of the invention comprises:

a) reaction of the urea derivative of formulae IIa, IIb, IIc, IId, IIe,IIf and IIg to give a diketopiperazine of the formula (III)

b) reaction of the diketopiperazine to give DL-methionyl-DL-methionine.This process includes methods A, B, C and D shown in scheme 3. In thisprocess, diketopiperazine (III) is formed as intermediate.

It is preferred in this embodiment, that the reaction of the ureaderivatives to give the diketopiperazine may be carried out at atemperature of from 50° C. to 200° C., preferably from 100° C. to 180°C. and particularly preferably from 140° C. to 170° C. In a preferredembodiment of this process, the reaction of the urea derivative to givethe diketopiperazine takes place under pressure, preferably under apressure of from 3 to 20 bar, more preferably 4 to 18 bar, andparticularly preferably under a pressure of from 6 to 15 bar.

The reaction of the urea derivative to give the diketopiperazinepreferably takes place in the presence of a base. The base in thisconnection may be selected from the group of nitrogen-containing bases,NH₄HCO₃, (NH₄)₂CO₃, KHCO₃, K₂CO₃, NH₄OH/CO₂ mixture, carbamate salts,alkali metal and alkaline earth metal bases. In a further preferredprocess, the reaction of the urea derivative to give thediketopiperazine takes place by reaction with methionine. A ratio ofurea derivative to methionine of from 1:100 to 1:0.5 may be preferred inthis embodiment.

In an additional further preferred process, the reaction of thediketopiperazine to give DL-methionyl-DL-methionine takes place byacidic hydrolysis. The acidic hydrolysis is in this case carried out inthe presence of an acid which is preferably selected from the group ofmineral acids, HCl, H₂CO₃, CO₂/H₂O, H₂SO₄, phosphoric acids, carboxylicacids and hydroxy carboxylic acids.

In another embodiment of the process of the invention, the reaction ofthe diketopiperazine to give DL-methionyl-DL-methionine may be by basichydrolysis. In this case, the basic hydrolysis is preferably carried outat a pH of from 7 to 14, particularly preferably at a pH of from 9 to12, very particularly preferably at a pH of from 10 to 11, in order toobtain DL-methionyl-DL-methionine. It may be moreover possible for thebasic conditions to be adjusted by using a substance which is preferablyselected from the group of nitrogen-containing bases, NH₄HCO₃,(NH₄)₂CO₃, NH₄OH/CO₂ mixture, carbamate salts, KHCO₃, K₂CO₃, carbonates,alkali metal and alkaline earth metal bases.

The acidic or basic hydrolysis may preferably be carried out attemperatures of from 50° C. to 200° C., preferably from 80° C. to 180°C. and particularly preferably from 90° C. to 160° C.

In a further embodiment, the reaction of the diketopiperazine to giveDL-methionyl-DL-methionine is carried out by introducing CO₂ into abasic solution, preferably into a basic ammonium hydroxide, potassiumhydroxide or sodium hydroxide solution.

In a preferred embodiment of the process of the present invention, thediketopiperazine may be isolated before the hydrolysis. According tothis embodiment, the diketopiperazine may be isolated by crystallizationfrom the reaction solution, preferably at a temperature of from −30 to120° C., more preferably at a temperature of 0 to 90° C. andparticularly preferably at a temperature of from 10 to 70° C.

To isolate the mixture of DD/LL/DL/LD-methionylmethionine diastereomersfrom basic reaction solutions, the solutions are acidified and themethionylmethionine may be obtained by crystallization or precipitation.It is preferred that the crystallization or precipitation be at a pHfrom 5 to 9, particularly preferred for the pH to be from 5 to 7, andvery particularly preferred for the pH to be about 5.6. It may bepossible in this to employ acids preferably from the group of mineralacids, HCl, H₂CO₃, CO₂/H₂O, H₂SO₄, phosphoric acids, carboxylic acidsand hydroxy carboxylic acids for the acidification.

To isolate the mixture of DD/LL/DL/LD-methionylmethionine diastereomersfrom acidic reaction solutions, bases may be added to neutralize thereaction solutions, and the methionylmethionine may be obtained bycrystallization or precipitation. It is preferred in this connection forthe pH to be from 5 to 9, particularly preferred for the pH to be from 5to 7, and very particularly preferred for the pH to be about 5.6. Thebases used in this case for the neutralization are preferably from thegroup of NH₄HCO₃, (NH₄)₂CO₃, nitrogen-containing bases, NH₄OH, carbamatesalts, KHCO₃, K₂CO₃, carbonates, alkali metal and alkaline earth metalbases.

In a special embodiment of the present invention, a process forfractionating the mixture of DD/LL/DL/LD-methionylmethioninediastereomers by fractional crystallization, thus obtaining the twopairs of enantiomers DD/LL-methionylmethionine andDL/LD-methionylmethionine is provided.

In a preferred embodiment of the process of fractional crystallizationof the present invention by acidification, the procedure includes:acidification of the DD/LL/DL/LD-methionylmethionine-containingsuspension until a clear solution is obtained; stepwise addition of abase to the acidic solution until a precipitate ofDL/LD-methionylmethionine is obtained; and DD/LL-methionylmethionine isobtained from the mother liquor. It is particularly preferred in thisembodiment, for the acidification to take place with an acid and for apH of from 0.1 to 1.0, preferably a pH of about 0.6, to be set, and forthe resulting clear solution subsequently to be adjusted with a base toa pH of from 5 to 6, preferably to a pH of about 5.6. It is possible touse as acid in this connection mineral acids, preferably phosphoricacid, sulfuric acid, hydrochloric acid or carbonic acid, or carbondioxide, and/or carboxylic acids, especially the C₁-C₄ carboxylic acidsformic acid, acetic acid, propionic acid, butyric acid or isobutyricacid. Carbonic acid or carbon dioxide may be particularly preferred asacids. It may be possible according to this embodiment for the carbonicacid or carbon dioxide to be introduced into the reaction mixture underatmospheric pressure or under superatmospheric pressure.

The basic conditions are obtained by adding a base selected from thegroup of consisting of NH₄HCO₃, (NH₄)₂CO₃, nitrogen-containing bases,NH₄OH, carbamate salts, KHCO₃, K₂CO₃, carbonates, alkali metal bases andalkaline earth metal bases. In a further preferred embodiment of theprocess of fractional crystallization by basification, the procedure maycomprise:

basification of the DD/LL/DL/LD-methionylmethionine-containingsuspension until a clear solution is obtained;stepwise addition of an acid to the basic solution to obtain aprecipitate of DL/LD-methionylmethionine;removing the DL/LD-methionylmethionine andobtaining DD/LL-methionylmethionine from the mother liquor.

It may be particularly preferred in this connection for the basificationto take place with a base and for a pH of from 7.5 to 14, preferably apH of about 9 to 13, to beobtained, and for the resulting clear solutionsubsequently to be adjusted with an acid to a pH of from 5 to 6,preferably to a pH of about 5.6. Bases preferably used in this case arebases from the group NH₄HCO₃, (NH₄)₂CO₃, nitrogen-containing bases,NH₄OH, carbamate salts, KHCO₃, K₂CO₃, carbonates, alkali metal andalkaline earth metal bases.

The acidic conditions of the acidification are preferably adjusted byusing an acid from the group of mineral acids, preferably phosphoricacid, sulfuric acid, hydrochloric acid, or carbonic acid or carbondioxide, and/or from the group of carboxylic acids, in particular theC₁-C₄ carboxylic acids formic acid, acetic acid, propionic acid, butyricacid and isobutyric acid. Carbonic acid or carbon dioxide may beparticularly preferably used.

In a preferred embodiment of the process of fractional crystallization,the temperature of the crystallization mixture is from 0° C. to 100° C.,preferably 5° C. to 60° C. and particularly preferably from 10° C. to40° C.

The resulting DD/LL-methionylmethionine can moreover be racemized andintroduced into the separation process described above, thus separatingthe two pairs of enantiomers DD/LL-methionylmethionine andDL/LD-methionylmethionine from one another.

All the processes of the present invention may be preferably carried outin an aqueous medium.

The processes of the present invention may furthermore be carried out inthe batch process known to the skilled worker or in continuousprocesses.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only, and are not intended to belimiting unless otherwise specified.

EXAMPLES A) Overview of the Individual Steps and Methods of the Processof the Invention

The process of the invention for preparing DL-methionyl-DL-methionine(I) and the separation into the DD/LL-I and DL/LD-I pairs ofdiastereomers are described in detail below.

The process of the invention for preparing DL-methionyl-DL-methionine(I) starts from a compound of the general formula II

where

IIa: R¹=COOH, R²=NHCONH₂ IIb: R¹=CONH₂, R²=NHCONH₂ IIc: R¹=CONH₂, R²=NH₂IId: R¹—R²=—CONHCONH— IIe: R¹=CN, R²=OH IIf: R¹=CN, R²=NH₂ IIg: R¹==O,R²=H.

This compound is transformed by various synthetic methods (A, B, C, D,E, F, G, H and J) into DL-methionyl-DL-methionine (I) (see scheme 3). Inmethods A, B, C, and D therein, the corresponding diketopiperazine (III)is produced as intermediate. In synthetic methods G, H and J, methioninehydantoin is produced as intermediate and is transformed directly intoDL-methionyl-DL-methionine (I). It is subsequently possible byfractional crystallization by method K to separate the two pairs ofdiastereomers DD/LL-I and DL/LD-I (see scheme 3).

B) Synthesis Examples Example 1 Synthesis of3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) from N-carbamoylmethionine (IIa) ByMethod A

17.5 g (90.0 mmol, purity: 99%) of N-carbamoylmethionine (IIa) weredissolved in 150 ml of water and stirred in a 200 ml Roth steelautoclave with magnetic stirring at 160° C. for 6 hours. The pressureincreased during this period. From time to time, gas was repeatedlydischarged until a pressure of 7 bar was reached. After completion ofthe reaction, the autoclave was cooled in an ice bath. The resultingsuspension was then filtered, and the filtered solid was washed severaltimes with water and dried in a drying oven at 50° C. in vacuo. Theisolated yield was 8.1 g (30.9 mmol) (69%) ofbis[2-(methylthio)ethyl]-2,5-piperazinedione (III), yellowish whitecrystals, purity>98% (HPLC), melting point 234-236° C.

¹H-NMR of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III) (500MHz, D₆-DMSO): δ=1.85-2.05 (m, 4H, 2×SCH₂CH₂); 2.049 (s, 6H, 2×SCH₃);2.46-2.60 (m, 4H, 2×SCH₂); 3.92-3.99 (m, 2H, 2×CH); 8.213 (s, 2H, 2×NH)

¹³C-NMR of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III) (125.8MHz, D₆-DMSO): δ=14.35 (CH₃); 14.38 (CH₃); 28.50 (CH₂S); 28.68 (CH₂S);31.92 (CH₂CH₂S); 32.33 (CH₂CH₂S); 52.92 (CH); 52.96 (CH); 167.69 (C═O);167.71 (C═O)

Elemental analysis for C₁₀H₁₈N₂O₂S₂ (M=262.39 g/mol): Calculated: C45.77; H 6.91; N 10.68; S 24.44 Found: C 45.94; H 6.96; N 10.64; S 24.38

Example 2 Synthesis of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione(III) (methioninediketopiperazine, DKP) from2-[(aminocarbonyDamino]-4-(methylthio)butanoamide(N-carbamoylmethioninamide) (IIb) By Method A

17.4 g (90.0 mmol, purity: 98.5%) of2-[(aminocarbony)amino]-4-(methylthio)butanoamide (IIb) were dissolvedin 150 ml of water and stirred in a 200 ml Roth steel autoclave withmagnetic stirring at 160° C. for 7 hours. The pressure increased duringthis heating. From time to time, gas was repeatedly discharged until apressure of 7 bar was reached. After completion of the reaction, theautoclave was cooled in an ice bath. The resulting suspension was thenfiltered, and the filtered solid was washed several times with water anddried in a drying oven at 50° C. in vacuo. The isolated yield was 9.2 g(35.1 mmol) (78%) of bis[2-(methylthio)ethyl]-2,5-piperazinedione (III),yellowish white crystals, purity>98% (HPLC).

The melting point and the NMR data agreed with those of example 1.

Example 3 Synthesis of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione(III) (methioninediketopiperazine, DKP) from5-[2-(methylthio)ethyl]-2,4-imidazolidinedione (IId)(methioninehydantoin) By Method A and Subsequent Reuse of the MotherLiquor (Cascade Reaction)

First Batch:

A suspension of 13.4 g (0.09 mol) of methionine, 17.2 g (0.09 mol,purity: 91%) of methioninehydantoin (IId) and 150 g of water wereintroduced into a 200 ml Roth steel autoclave with magnetic stirring andstirred at 160° C. for 6 hours, during which the pressure increased to15 bar. From time to time, the autoclave was decompressed until thepressure settled at a constant 10 bar. The autoclave was then cooled inan ice bath, and the resulting suspension was filtered and the solid waswashed with 75 ml of water. Finally, the solid was dried in a vacuumdrying oven at 50° C. overnight.Bis[2-(methylthio)ethyl]-2,5-piperazinedione (III) was isolated asyellowish white crystals.

Subsequent Batches:

The washing water and the mother liquor from the preceding batch werecombined and concentrated to 90 ml in a rotary evaporator at 50° C. 17.2g (0.09 mol, purity: 91%) of methioninehydantoin (IId) were taken upwith the concentrated mother liquor and made up to 150 g of solutionwith water. The resulting solution was introduced into a 200 ml Rothsteel autoclave with magnetic stirring and stirred at 160° C. for 6hours, during which the pressure increased to 15 bar. From time to time,the autoclave was decompressed until the pressure remained constant at10 bar. Further working up took place as described for the first batch.

Example 4 Synthesis of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione(III) (methioninediketopiperazine, DKP) from2-amino-4-(methylthio)butanoamide (methioninamide) (IIc) By Method B

16.6 g (0.09 mol) of 2-amino-4-(methylthio)butanoamide hydrochloride(IIc) and 8.7 g (0.09 mol) of (NH₄)₂CO₃ were dissolved in 150 g of waterand stirred in a 200 ml Roth steel autoclave with magnetic stirring at160° C. for 6 hours. The autoclave was then cooled in an ice bath. Theresulting suspension was then filtered, and the filtered solid waswashed several times with water and dried in a drying oven at 50° C. invacuo. The isolated yield was 6.5 g (24.8 mmol) (55%) ofbis[2-(methylthio)ethyl]-2,5-piperazinedione (III), yellowish whitecrystals, purity>98% (HPLC).

The melting point and the NMR data agreed with those from example 1.

Example 5 Synthesis of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione(III) (methioninediketopiperazine, DKP) from2-hydroxy-4-(methylthio)butanenitrile (3-(methylmercapto)propionaldehydecyanohydrin, MMP-CH) (IIe) By Method C

A solution of 30.5 g (0.232 mol) of2-hydroxy-4-(methylthio)butanenitrile (IIe) and 360 g of water wasslowly added dropwise at RT to a suspension of 22.4 g (0.283 mol=1.22eq.) of NH₄HCO₃ in 20 g of water and stirred for 2 h. The NH₄HCO₃dissolved during this time. The resulting solution was subsequentlystirred at 50° C. for 7 h and then at room temperature overnight. Thereaction mixture was then transferred into a 500 ml steel autoclave,heated to 160° C., and stirred at this temperature for 6 hours. Theautoclave was then cooled in an ice bath, the resulting suspension wasfiltered, and the solid was washed with 50 ml of water. Finally, thepale solid was dried in a vacuum drying oven at 50° C. overnight. Theisolated yield was 17.8 g (67.8 mmol) (58%) ofbis[2-(methylthio)ethyl]-2,5-piperazinedione (III), yellowish whitecrystals, purity>98% (HPLC).

The melting point and the NMR data agreed with those from example 1.

Example 6 Synthesis of 3 ,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III) (methioninediketopiperazine, DKP) From2-amino-4-(methylthio)butanenitrile (methioninenitrile) (IIf) By MethodC

A moderate stream of CO₂ was passed into a solution of 26.2 g (0.201mol) of 2-amino-4-(methylthio)butanenitrile (IIf) in 330 g of water overa period of 3 hours, during which the temperature rose to 45° C. and thepH settled at 8. Stirring was then continued at room temperatureovernight. The next morning, the reaction mixture was transferred into a500 ml steel autoclave, heated to 160° C. and stirred at thistemperature for 6 hours. The autoclave was then cooled in an ice bath,the resulting suspension was filtered, and the solid was washed with 50ml of water and dried in a vacuum drying oven at 50° C. overnight. Theisolated yield was 15.7 g (59.7 mmol) (59%) ofbis[2-(methylthio)ethyl]-2,5-piperazinedione (III), yellowish whitecrystals, purity>98% (HPLC).

The melting point and the NMR data agreed with those from example 1.

Example 7 Synthesis of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione(III) (methioninediketopiperazine, DKP) From3-(methylthio)propanaldehyde (3-(methylmercapto)propionaldehyde, MMP)(IIg) By Method D

66.0 g (0.68 mol) of (NH₄)₂CO₃ were introduced into 100 g of water andcooled to 5° C. in an ice bath. Then, over the course of 25 minutes,16.6 g (0.61 mol) of freshly distilled hydrocyanic acid were addeddropwise, during which the temperature of the suspension was kept in therange from 5 to 10° C. Addition of 860 g of water was followed bydropwise addition, at 10° C., of 60.3 g (0.58 mol) of3-(methylthio)propionaldehyde (IIg) over a period of 80 min. The pHremained constant in the range from 8.5 to 9 during this. The reactionmixture was then heated to 50° C. and stirred at this temperature for 7hours. After completion of the reaction, the reaction mixture was cooledto 5° C. in an ice bath and stored in a refrigerator overnight. The nextmorning, the mixture was transferred into a 21 steel autoclave, heatedto 160° C. and stirred at this temperature for 6 hours. The autoclavewas then cooled in an ice bath, the resulting suspension was filteredand washed with 150 ml of water, and the solid was dried in a vacuumdrying oven at 50° C. overnight. The isolated yield was 48.6 g (185.2mmol) (64%) of bis[2-(methylthio)ethyl]-2,5-piperazinedione (III),yellowish white crystals, purity>98% (HPLC).

The melting point and the NMR data agreed with those from example 1.

Example 8 Synthesis of DD/LL/DL/LD-methionylmethionine (I) from3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) with Concentrated Hydrochloric Acid ByMethod E

655.9 g (2.50 mol) of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione(III) (DKP) were suspended in 1661 g of water. While stirring, 271.0 gof conc. hydrochloric acid were very slowly added dropwise and thencautiously heated, with very vigorous stirring, to reflux. Severefoaming may occur during this. The reaction mixture was heated to refluxfor 5.5 hours, thus dissolving all the solid. During the subsequentcooling, unreacted DKP (III) precipitated and was filtered off. This DKPmay be employed again for further hydrolyses in later reactions. Thefiltrate was then adjusted to pH 6 in a glass beaker in an ice bath with32% strength aqueous ammonia. A DD/LL/DL/LD-methionylmethionine (I)separates out as a thick mass of crystals, and 50:50 mixture of the twopairs of diastereomers (DL/LD-Met-Met) (DL/DL-I) and (DD/LL-Met-Met)(DD/LL-I) during this. It was finally dried in a drying oven at 60° C.in vacuo. Yield: 601.0 g (2.14 mol) (85.7%) ofDD/LL/DL/LD-methionylmethionine (I), slightly yellowish solid, purity98% (HPLC).

¹H-NMR of DD/LL/DL/LD-methionylmethionine (I) (500 MHz, D₆-DMSO+HCl):δ=1.86-2.16 (m, 4H, 2×SCH₂CH₂); 2.050 (s, 3H, SCH₃); 2.060 (s, 3H,SCH₃); 2.44-2.64 (m, 4H, 2×SCH₂); 2.90-4.00 (m, 1H, CH); 4.32-4.42 (m,1H, CH); 8.45 (bs, 3H, NH₃ ⁺); 8.98-9.08 (m, 1H, 2×NH)

¹³C-NMR of DD/LL/DL/LD-methionylmethionine (I) (125.8 MHz, D₆-DMSO+HCl):δ=14.33 (CH₃); 14.38 (CH₃); 27.74; 27.94; 29.51; 30.04; 30.13; 30.89;30.95; 51.00; 51.29; 51.54 (CH, CH₂); 168.05 (CONH); 168.19 (CONH);172.55 (COOH); 172.62 (COOH)

Elemental analysis for C₁₀H₂₀N₂O₃S₂ (M=280.41 g/mol): Calculated: C42.83; H 7.19; N 9.99; S 22.87 Found: C 42.61; H 7.19; N 10.06; S 22.72

Example 9 Industrial synthesis of DD/LL/DL/LD-methionylmethionine (I)from 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) With Concentrated Hydrochloric Acid ByMethod E

500 l of water were introduced into a 500 l enameled tank with stirrer,32 l of concentrated hydrochloric acid and 78.6 kg of3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III) (DKP) were added,and the apparatus was closed tightly. It was then heated at 110° C.while stirring for 2 hours, during which the pressure rose to 2.5 barand the DKP (III) virtually completely dissolved. After the reaction wascomplete, the mixture was cooled to 20° C., and the unreacted DKP wasspun down in a centrifuge. The solid was washed with 10 l of water. Thefiltrate and washing water were then collected in an 800 l container andsubsequently introduced into a 500 l tank with stirrer again. Additionof 2 kg of activated carbon was followed by stirring at 20° C. for 30min. The suspension was then filtered through a filter press into afurther 500 l tank with stirrer. About 28 l of concentrated ammoniasolution were then added to precipitate at pH 6 theDD/LL/DL/LD-methionylmethionine (I). During this there was an initialpreferential precipitation of the less soluble racemic pair ofdiastereomers DL/LD-methionylmethionine (DL/LD-I). This was spun downand the mother liquor was concentrated together with washing water toone quarter of the original volume in vapor pump vacuum at an internaltemperature not exceeding 40° C. During this, the more soluble racemicpair of diastereomers DD/LL-methionylmethionine (DD/LL-I) crystallizedtogether with small amounts of the slightly soluble DL/LD-I. Completionof the distillation was followed by cooling to 20° C. andcentrifugation. The separated mother liquor and washing water werediscarded. Both fractions were dried in vacuo at 70° C. In total, it waspossible to obtain 64.2 kg (78%) of DD/LL/DL/LD-methionyl-methionine (I)as mixture of diastereomers. Purity>98% (HPLC).

The melting point and the NMR data agreed with those from example 8.

Example 10 Synthesis of DD/LL/DL/LD-methionylmethionine (I) from3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) Under Alkaline Conditions, e.g. WithAmmonia By Method F

65.6 g (0.25 mol) of 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione(III) (DKP), 70 ml of 25% strength ammonia solution and 500 ml of waterwere heated at 150° C. in an autoclave for 2 hours. After cooling, theunreacted DKP (III) (16.0 g=24.4%) was filtered off with suction. Thiscan be employed again in a subsequent batch. The filtrate wasconcentrated in a rotary evaporator at a water temperature of 80-90° C.until the first crystals separated out. After cooling and leaving tostand overnight it was possible to isolate after filtration and dryingin total 49.3 g (70.3%) of DD/LL/DL/LD-methionylmethionine (I) as 50:50mixture of the two pairs of diastereomers DL/DL-I and DD/LL-I as a whitesolid. Purity 98% (HPLC).

The melting point and the NMR data agreed with those from example 8.

Example 11 Purification of DD/LL/DL/LD-methionylmethionine (I)

500 g of DD/LL/DL/LD-methionylmethionine (I) were suspended in 7800 g ofdeionized water (pH 5.3). At 26° C., the pH was adjusted to 1.0 with346.6 g of 50% by weight sulfuric acid. The methionylmethioninedissolved completely. For clarification, 18 g of activated carbon wereadded to the yellowish turbid solution and stirred for 60 minutes. Theactivated carbon was filtered off, and the clear colorless solution wasadjusted to pH 5.6 with 228 g of 32% by weight ammonia solution. Thesolution was left to stand overnight. The precipitated white solid wasfiltered off with suction and dried in a drying oven at 50° C. in vacuo.Yield: 460.5 g (92%) of DD/LL/DL/LD-methionylmethionine (I), brilliantwhite solid, purity>99% (HPLC).

The NMR data agreed with those from example 8.

Example 12 Synthesis of DD/LL/DL/LD-methionylmethionine (I) fromN-carbamoylmethionine (IIa) and DL-methionine with KOH By Method G

13.4 g (0.09 mol) of DL-methionine, 17.5 g (0.09 mol, purity: 99%) ofN-carbamoylmethionine (IIa) and 11.9 g (0.18 mol) of 85% pure KOH weredissolved in 150 ml of water and stirred at 150° C. in a 200 ml Rothsteel autoclave with magnetic stirring for 5 hours, during which thepressure increased to 6 bar. After reaction was complete, the autoclavewas cooled, and the precipitated3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) was filtered off and washed with alittle water. The washing water and the mother liquor were combined andconcentrated to a volume of 130 ml in a rotary evaporator at 40° C. Amoderate stream of CO₂ was then passed into the resulting solution untila pH of 6.4 was reached and a white solid precipitated. This wasfiltered off, washed with a little cold water and dried in a vacuumdrying oven at 50° C. overnight. The isolated yield was 11.4 g (40.6mmol) (45%) of DD/LL/DL/LD-methionylmethionine (I), white solid,purity >98% (HPLC).

The NMR data agreed with those from example 8.

Example 13 Synthesis of DD/LL/DL/LD-methionylmethionine (I) from5-[2-(methylthio)ethyl]-2,4-imidazolidinedione (IId)(methioninehydantoin) and DL-methionine with KOH By Method G

13.4 g (0.09 mol) of DL-methionine, 17.2 g (0.09 mol, purity: 91%) ofmethioninehydantoin (IId) and 8.9 g (0.135 mol) of 85% pure KOH weredissolved in 150 ml of water and stirred at 150° C. in a 200 ml Rothsteel autoclave with magnetic stirring for 5 hours, during which thepressure increased to 8 bar. After the reaction was complete, theautoclave was cooled, the resulting suspension was filtered and theprecipitated 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) was washed several times with a littlewater. Mother liquor and washing water were combined, and the resultingsolution was concentrated to a volume of 125 ml in a rotary evaporatorat 40° C. The concentrate was cautiously neutralized with concentratedhydrochloric acid. A white solid precipitated on stirring at roomtemperature and at a pH of 5.8 overnight. This solid was filtered off,washed with a little cold water and dried in a vacuum drying oven at 50°C. overnight. The isolated yield was 17.5 g (62.4 mmol) (69%) ofDD/LL/DL/LD-methionylmethionine (I), white solid, purity>98% (HPLC).

The NMR data agreed with those from example 8.

Example 14 Synthesis of DD/LL/DL/LD-methionylmethionine (I) from5-[2-(methylthio)ethyl]-2,4-imidazolidinedione (IId)(methioninehydantoin) and DL-methionine with K₂CO₃ By Method G

13.4 g (0.09 mol) of DL-methionine, 17.2 g (0.09 mol, purity: 91%) ofmethioninehydantoin (IId) and 12.4 g (0.09 mol) of K₂CO₃ were dissolvedin 150 ml of water and stirred at 150° C. in a 200 ml Roth steelautoclave with magnetic stirring for 5 hours, during which the pressureincreased to 12 bar. After the reaction was complete, the autoclave wascooled, and the precipitated3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) was filtered off and washed with alittle water. The washing water and the mother liquor were combined andconcentrated to a volume of 135 ml in a rotary evaporator at 40° C. Amoderate stream of CO₂ was then passed into the resulting solution untila pH of 6.8 was reached and a white solid precipitated. This wasfiltered off, washed with a little cold water and dried in a vacuumdrying oven at 50° C. overnight. Yield: 14.3 g (60.0 mmol) (57%) ofDD/LL/DL/LD-methionylmethionine (I), white solid, purity>99% (HPLC).

The NMR data agreed with those from example 8.

Example 15 Synthesis of DD/LL/DL/LD-methionylmethionine (I) from5-[2-(methylthio)ethyl]-2,4-imidazolidinedione (IId)(methioninehydantoin) and DL-methionine with KHCO₃ by Method G

13.4 g (0.09 mol) of DL-methionine, 17.2 g (0.09 mol, purity: 91%) ofmethioninehydantoin (IId) and 9.1 g (0.09 mol) of KHCO₃ were dissolvedin 150 ml of water and stirred at 150° C. in a 200 ml Roth steelautoclave with magnetic stirring for 5 hours, during which the pressureincreased to 12 bar. After the reaction was complete, the autoclave wascooled, and the precipitated3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) was filtered off and washed with alittle water. The washing water and the mother liquor were combined andconcentrated to a volume of 120 ml in a rotary evaporator at 40° C. Amoderate stream of CO₂ was then passed into the resulting solution untila pH of 6.3 was reached and a white solid precipitated. This wasfiltered off, washed with a little cold water and dried in a vacuumdrying oven at 50° C. overnight. Yield: 16.0 g (57.1 mmol) (63%) ofDD/LL/DL/LD-methionylmethionine (I), white solid, purity>99% (HPLC).

The NMR data agreed with those from example 8.

Example 16 Synthesis of DD/LL/DL/LD-methionylmethionine (I) from2-amino-4-(methylthio)butanoamide (IIc) (methioninamide) andDL-methionine with (NH₄)₂CO₃ By Method H

8.3 g (0.045 mol) of 2-amino-4-(methylthio)butanoamide (IIc)hydrochloride, 6.7 g (0.045 mol) of methionine, 4.3 g (0.045 mol) of(NH₄)₂CO₃ and 3.0 g (0.045 mol) of 85% pure KOH were dissolved in 75 gof water and stirred at 160° C. in a 200 ml Roth steel autoclave withmagnetic stirring for 6 hours. The autoclave was then cooled in an icebath, the resulting suspension was filtered off, and the precipitated3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) was washed with a little water. Thewashing water and the mother liquor were combined and concentrated to avolume of 70 ml in a rotary evaporator at 40° C. A moderate stream ofCO₂ was then passed into the resulting solution until a pH of 6.3 wasreached and a white solid precipitated. This was filtered off, washedwith a little cold water and dried in a vacuum drying oven at 50° C.overnight. Yield: 7.8 g (27.8 mmol) (62%) ofDD/LL/DL/LD-methionylmethionine (I), white solid, purity>98% (HPLC).

The NMR data agreed with those from example 8.

Example 17 Synthesis of DD/LL/DL/LD-methionylmethionine (I) from2-hydroxy-4-(methylthio)butanenitrile (IIe)(3-(methylmercapto)propionaldehyde cyanohydrin, MMP-CH) andDL-methionine with NH₄HCO₃ By Method H

15.2 g (0.116 mol) of 2-hydroxy-4-(methylthio)butanenitrile (IIe) wereslowly added dropwise at RT to a suspension of 11.1 g (0.141 mol=1.22eq.) of NH₄HCO₃ in 10 g of water and stirred for 2 h. The NH₄HCO₃dissolved during this. Then 180 g of water were added and the resultingsolution was stirred at 50° C. for 7 h and at room temperatureovernight. The next morning, 17.3 g (0.116 mol) of methionine, 7.7 g(0.116 mol) of 85% pure KOH and a further 180 g of water were added, andthe reaction mixture was transferred into a 11 steel autoclave, heatedto 160° C. and stirred at this temperature for 6 hours. The autoclavewas then cooled in an ice bath, the resulting suspension was filtered,and the precipitated 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione(III) (methioninediketopiperazine, DKP) was washed with 100 ml of water.Mother liquor and washing water were combined, and the resultingsolution was concentrated to a volume of 160 ml in a rotary evaporatorat 40° C. The concentrate was cautiously neutralized with 50% strengthsulfuric acid. A white solid precipitated on stirring at roomtemperature and at a pH of 5.4 overnight. This solid was filtered off,washed with a little cold water and dried in a vacuum drying oven at 50°C. overnight. Yield: 15.2 g (54.2 mmol) (47%) ofDD/LL/DL/LD-methionylmethionine (I), white solid, purity>99% (HPLC).

The NMR data agreed with those from example 8.

Example 18 Synthesis of DD/LL/DL/LD-methionylmethionine (I) from2-amino-4-(methylthio)butanenitrile (IIf) (methioninenitrile) with CO₂and DL-methionine By Method H

A moderate stream of CO₂ was passed into a solution of 26.2 g (0.201mol) of 2-amino-4-(methylthio)butanenitrile (IIf) in 330 g of water overa period of 3 hours, during which the temperature rose to 45° C. and thepH settled at 8. Stirring was then continued at room temperatureovernight. The next morning, the reaction mixture was mixed with 30.0 g(0.201 mol) of methionine and 13.3 g (0.201 mol) of 85% pure KOH andtransferred into a 1 l steel autoclave, heated to 160° C. and stirred atthis temperature for 6 hours. The autoclave was then cooled in an icebath, the resulting suspension was filtered, and the precipitated3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) was washed with a little water. Thewashing water and the mother liquor were combined and concentrated to avolume of 280 ml in a rotary evaporator at 40° C. A moderate stream ofCO₂ was then passed into the resulting solution until a pH of 6.0 wasreached and a white solid precipitated. This was filtered off, washedwith a little cold water and dried in a vacuum drying oven at 50° C.overnight. Yield: 32.7 g (116.6 mmol) (58%) ofDD/LL/DL/LD-methionylmethionine (I), white solid, purity >98% (HPLC).

The NMR data agreed with those from example 8.

Example 19 Synthesis of DD/LL/DL/LD-methionylmethionine (I) From3-(methylthio)propanaldehyde (IIg) (MMP) with Hydrocyanic Acid, AmmoniumCarbonate and DL-methionine By Method J

66.0 g (0.68 mol) of (NH₄)₂CO₃ were introduced into 100 g of water andcooled to 5° C. in an ice bath. Then 16.55 g (0.612 mol) of freshlydistilled hydrocyanic acid were added dropwise over the course of 25min, during which the temperature of the suspension was kept at 5 to 10°C. After 500 g of water had been added, 60.3 g (0.58 mol) of3-(methylthio)propionaldehyde (IIg) were added dropwise at 10° C. over aperiod of 80 min. The pH remained constant in the range from 8.5 to 9during this. The reaction mixture was then heated to 50° C. and stirredat this temperature for 7 hours. After the reaction was complete, thereaction mixture was cooled to 5° C. in an ice bath and stored in arefrigerator overnight. The next morning, 86.5 g (0.58 mol)2-amino-4-(methylthio)butanoic acid (methionine), 38.3 g (0.58 mol) of85% pure KOH (0.58 mol), and a further 530 g of water were added. Themixture was transferred into a 21 steel autoclave, heated to 160° C. andstirred at this temperature for 6 hours. The autoclave was then cooledin an ice bath, the resulting suspension was filtered, and theprecipitated 3,6-bis[2-(methylthio)ethyl]-2,5-piperazinedione (III)(methioninediketopiperazine, DKP) was washed with a little water. Thewashing water and the mother liquor were combined and concentrated to avolume of 800 ml in a rotary evaporator at 40° C. A moderate stream ofCO₂ was then passed into the resulting solution until a pH of 6.0 wasreached and a white solid precipitated. This was filtered off, washedwith a little cold water and dried in a vacuum drying oven at 50° C.overnight. Yield: 85.1 g (0.30 mol) (52%) ofDD/LL/DL/LD-methionylmethionine (I), white solid, purity>98% (HPLC).

The NMR data agreed with those from example 8.

Example 20 Separation of the Two Pairs of DiastereomersDD/LL-methionylmethionine (DD/LL-I) and DL/LD-methionylmethionine(DL/LD-I) by Fractional Crystallization fromDD/LL/DL/LD-methionylmethionine (I) by Method K

a) DL/LD-Methionylmethionine (DL/LD-I):

290.4 g of DD/LL/DL/LD-methionylmethionine (I) (50:50 mixture of DD/LL-Iand DL/LD-I) were suspended in 2614 g of deionized water and adjusted topH 0.6 with 381.7 g of 50% by weight sulfuric acid. The clear colorlesssolution was adjusted to pH 5.6 with 265.9 g of 32% by weight ammoniasolution, and the resulting white precipitate was filtered off withsuction (580.9 g moist). The solid was finally dried in a drying oven invacuo at 50° C. The yield was 126.2 g (86.9%) ofDL/LD-methionylmethionine (DL/LD-I), white solid, purity>98% (HPLC),melting range 232-233° C. (decomp.).

¹H-NMR of DL/LD-methionylmethionine (DL/LD-I) (500 MHz, D₆-DMSO+HCl):1.88-2.12 (m, 4H, 2×SCH₂CH₂); 2.031 (s, 3H, CH₃); 2.041 (s, 3H, CH₃);2.48-2.56 (m, 4H, 2×SCH₂); 3.87-3.95 (m, 1H, CH); 4.30-4.38 (m, 1H, CH);8.429 (d, 3H, ³J=4.4 Hz, NH₃ ⁺); 9.034 (d, 1H, ³J=8.0 Hz, NH)

¹³C-NMR of DL/LD-methionylmethionine (DL/LD-I) (125.8 MHz, D₆-DMSO+HCl):14.57 (CH₃); 14.62 (CH₃); 28.19; 29.75; 30.28; 31.19; 51.25 (CH); 51.79(CH); 168.29 (CONH); 172.80 (COOH)

Solubility (water, 20° C.): 0.4 g/l

b) DD/LL-Methionylmethionine (DD/LL-I):

The colorless mother liquor from a) was concentrated in a rotaryevaporator at 35° C. under water pump vacuum. A white suspension wasobtained. The white solid composed of ammonium sulfate, residues ofDL/LD-I and target compound was then filtered off with suction and driedin vacuo at 50° C. The three solids were separated by suspending themixture in deionized water and stirring. The undissolved DL/LD-I wasfiltered off with suction, and the mother liquor was concentrated toabout one fifth in a rotary evaporator at 50° C. under water pumpvacuum. After prolonged standing, DD/LL-methionylmethionine (DD/LL-I)crystallized as a white solid. It was finally filtered off with suctionand dried in a vacuum drying oven at 50° C. The yield was 78.2 g (53.9%)based on DD/LL-methionylmethionine (DD/LL-I), white solid, >96% (HPLC),melting range 226-227° C. (decomposition).

¹H-NMR of DD/LL-methionylmethionine (DD/LL-I) (500 MHz, D₆-DMSO+HCl):1.84-2.12 (m, 4H, 2×SCH₂CH₂); 2.044 (s, 3H, CH₃); 2.046 (s, 3H, CH₃);2.48-2.62 (m, 4H, 2×SCH₂); 3.89-3.97 (m, 1H, CH); 4.33-4.40 (m, 1H, CH);8.422 (d, 3H, ³J=4.0 Hz, NH₃ ⁺); 9.065 (d, 1H, ³J=7.5 Hz, NH)

¹³C-NMR of DD/LL-methionylmethionine (DD/LL-I) (125.8 MHz, D₆-DMSO+HCl):14.56 (CH₃); 14.57 (CH₃); 27.97; 29.73; 30.35; 31.11; 51.22 (CH); 51.50(CH); 168.41 (CONH); 172.83 (COOH)

Solubility (water, 20° C.): 21.0 g/l

Example 21 Racemization of the Two Pairs of DiastereomersDD/LL-methionylmethionine (DD/LL-I) and DL/LD-methionylmethionine(DL/LD-I) Under Basic Conditions

a) Racemization of DL/LD-methionylmethionine (DL/LD-I)

12.6 g (45.0 mmol) of the pair of diastereomersDL/LD-methionylmethionine (DL/LD-I) were dissolved together with 3.1 g(22.5 mmol) of K₂CO₃ in 75 ml of water in a 200 ml Roth laboratoryreactor and heated to 160° C. while stirring. The pressure rose to 7 barduring this. After 6 hours at this temperature, the autoclave was cooledin an ice bath. The resulting suspension was then filtered, and thesolid was filtered off, washed several times with water and dried in adrying oven in vacuo at 50° C. The isolated yield was 6.5 g (24.8 mmol)(55%) of bis[2-(methylthio)ethyl]-2,5-piperazinedione (III), yellowishwhite crystals, purity>98%, melting point 234-236° C.; diastereomerratio: 52:48 (DD/LL-III:meso-III). The washing water and the motherliquor were combined and concentrated to a volume of 25 ml in a rotaryevaporator at 40° C. A moderate stream of CO₂ was then passed into theresulting solution until the pH reached 6.0 and a white solidprecipitated. This was filtered off, washed with a little cold water anddried in a vacuum drying oven at 50° C. overnight. The isolated yieldwas 5.7 g (20.3 mmol) (45%) of DD/LL/DL/LD-methionylmethionine (I),white solid, purity>98% (HPLC).

The NMR data agreed with those from example 8.

b) Racemization of DD/LL-methionylmethionine (DD/LL-I)

12.6 g (45.0 mmol) of DD/LL-methionylmethionine (DD/LL-I) were dissolvedtogether with 4.5 g (45.0 mmol) of KHCO₃ in 75 ml of water in a 200 mlRoth laboratory reactor and heated to 160° C. while stirring. Thepressure increased to 7 bar and, after 6 hours at this temperature, theautoclave was cooled in an ice bath. The resulting suspension was thenfiltered, and the filtered solid was washed several times with water anddried in a drying oven in vacuo at 50° C. The isolated yield was 6.0 g(22.9 mmol) (51%) of bis[2-(methylthio)ethyl]-2,5-piperazinedione (III),yellowish white crystals, purity>98% (HPLC), melting point 233-236° C.;diastereomer ratio: 54:46 (DD/LL-III : meso-III). The washing water andthe mother liquor were combined and concentrated to a volume of 25 ml ina rotary evaporator at 40° C. A moderate stream of CO₂ was then passedinto the resulting solution until the pH reached 6.0 and a white solidprecipitated. This was filtered off, washed with a little cold water anddried in a vacuum drying oven at 50° C. overnight. The isolated yieldwas 5.5 g (19.6 mmol) (44%) of DD/LL/DL/LD-methionylmethionine (I),white solid, purity>98% (HPLC).

The NMR data agreed with those from example 8.

Example 22

In Vitro Digestion Experiments on DL-methionyl-DL-methionine (I) WithDigestive Enzymes From Omnivorous Carp

a) Isolation of the Digestive Enzymes From Common Carp (Cyprinus carpiomorpha noblis)

The method for isolating the digestive enzymes was based on that of EIDand MATTY (Aquaculture 1989, 79, 111-119). For this purpose, theintestine of five one-year old common carp (Cyprinus carpio morphanoblis) was exposed, rinsed with water and cut open longitudinally, andin each case the intestinal mucosa was scraped off. This was comminutedtogether with crushed ice using a mixer. The resulting suspension wastreated with an ultrasonic probe in order to disrupt cells which werestill intact. The cell constituents and fat were separated bycentrifuging the suspension at 4° C. for 30 minutes, and the homogenatewas decanted off and sterilized with a trace of thimerosal. 260.7 ml ofenzyme solution from the intestinal mucosa were obtained from 5 commoncarp, and the solution was stored in the dark at 4° C.

b) Procedure for the In Vitro Digestion Investigations

DL-Methionyl-DL-methionine (I) and the corresponding pairs ofdiastereomers DD/LL-I and DL/LD-I were taken up in TRIS/HCl buffersolution and mixed with the enzyme solution. A blank was made up in eachcase without enzyme solution for comparison and to estimate the purelychemical cleavage rate. A sample was taken from time to time, and thecomposition thereof was detected and quantified with the aid of acalibrated HPLC. The conversion was determined as the quotient of thearea for methionine and the area for methionylmethionine (I) (see FIGS.6 and 7).

TABLE 1 Sample Blank Precharge Substrate 0.143 mmol 0.143 mmol Met-Met(I) (40.1 mg) (40.1 mg) TRIS/HCl buffer solution, 5.7 ml 8.3 ml pH 9.5Reaction start Enzyme solution 2.6 ml — ({circumflex over (=)} 5% carpsolution) Reaction 37° C. 37° C. Reaction stop 0.2 ml of reactionsolution was taken up in 9.8 ml of 10% strength H₃PO₄ solution.

Example 23 In Vitro Digestion Experiment on DL-methionyl-DL-methionine(I) with Digestive Enzymes From Carnivorous Trout

a) Isolation of the Digestive Enzymes From Rainbow Trout (Oncorhynchusmykiss)

The method for isolating the digestive enzymes was based on that of EIDand MATTY (Aquaculture 1989, 79, 111-119). For this purpose, theintestine of six one-year old rainbow trout (Oncorhynchus mykiss) wasexposed and processed as described in example 22.

b) Procedure for the In Vitro Digestion Investigations

The in vitro investigations were carried out in analogy to example 22(see FIGS. 8 and 9).

TABLE 2 Sample Blank Precharge Substrate 0.143 mmol 0.143 mmol Met-Met(I) (40.1 mg) (40.1 mg) TRIS/HCl buffer solution, 5.7 ml 9.8 ml pH 9.5Reaction start Enzyme solution 4.2 ml — ({circumflex over (=)} 10% troutsolution) Reaction 37° C. 37° C. Reaction stop 0.2 ml of reactionsolution was taken up in 9.8 ml of 10% strength H₃PO₄ solution.

Example 24 In Vitro Digestion Experiments on DL-methionyl-DL-methionine(I) with Digestive Enzymes From Omnivorous Shrimps

a) Isolation of the Digestive Enzymes from whiteleg shrimps (LitopenaeusVannamei)

The method for isolating the digestive enzymes was based on that ofEzquerra and Garcia-Carreno (J. Food Biochem. 1999, 23, 59-74). For thispurpose, the hepatopancreas was removed from five kilograms of whitelegshrimps (Litopenaeus Vannamei) and comminuted together with crushed iceusing a mixer. The further processing was carried out in analogy toexample 22.

b) Procedure for the In Vitro Digestion Investigations

The in vitro investigations were carried out in analogy to example 22(see FIGS. 10 and 11).

TABLE 3 Sample Blank Precharge Substrate 0.143 mmol 0.143 mmol Met-Met(I) (40.1 mg) (40.1 mg) TRIS/HCl buffer solution, 5.7 ml 7.9 ml pH 9.5Reaction start Enzyme solution 2.2 ml — ({circumflex over (=)} 2shrimps) Reaction 37° C. 37° C. Reaction stop 0.2 ml of reactionsolution was taken up in 9.8 ml of 10% strength H₃PO₄ solution.

Example 25 Biotransformation of D- to L-methionine with Enzymes FromIntestine, Liver and Pancreas of Common Carp

a) Isolation of the Digestive Enzymes from Common Carp (Cyprinus carpiomorpha noblis)

The method for isolating the digestive enzymes was based on that of EIDand MATTY (Aquaculture 1989, 79, 111-119). For this purpose, theintestine of five one-year old common carp (Cyprinus carpio morphanoblis) was exposed and processed as described in example 22. To isolateliver enzymes, the livers were isolated, homogenized and treated inanalogy to the processing of the intestinal enzymes in example 22. Theprocedure for enzyme isolation from the pancreas was also analogousthereto.

b) Procedure for the In Vitro Biotransformation of D- to L-methionine

D-Methionine was taken up in buffer solution, and the enzyme solutionwas added. A blank without enzyme solution was made up in each case ascomparison and for estimating the purely chemical transformation rate.After 24 hours, a sample was taken and the composition was detected andquantified with the aid of calibrated HPLC. The conversion wasdetermined as the quotient of the area for L-methionine and the area forD-methionine (see FIG. 4).

TABLE 4 Sample Blank Precharge Substrate 0.143 mmol 0.143 mmolD-Methionine (21.3 mg) (21.3 mg) Buffer solution 11.7 ml 23.4 mlReaction start Enzyme cocktail 11.7 ml — ({circumflex over (=)} 5% carpsolution) Reaction 37° C. 37° C. Reaction stop 0.2 ml of reactionsolution was taken up in 9.8 ml of 10% strength H₃PO₄ solution.

Buffer Solutions: Citrate buffer: pH 5, pH 6 and pH 7 Phosphate buffer:pH 8 TRIS/HCl buffer: pH 9

Enzyme cocktail composed of intestinal, hepatic and pancreatic enzymes({circumflex over (=)}5% carp solution): 2.6 ml of enzyme solution fromintestinal mucosa 3.5 ml of enzyme solution from liver 5.6 ml of enzymesolution from pancreas

Example 26 Leaching Characteristics of the Mixtures ofMethionylmethionine Diastereomers LL/DD/LD/DL-I, DD/LL-I and DL/LD-Ifrom Feed Pellets Compared with DL-methionine, MHA and Calcium MHA

Feed Mixture:

The feed matrix used was a methionine-deficient feed mixture ofconventional ingredients such as, for example, soybean meal, soybeanoil, cornstarch, wheat meal, fish meal, cellulose, crystalline essentialamino acids and minerals and vitamins as premixes. This mixture was thensupplemented batchwise in 20 kg batches in each case with the methioninederivatives stated in table 5, with a 0.25% supplementation rate (basedon sulfur equivalents), and was homogenized and then pelleted with steamtreatment. As comparison with methionylmethionine (I), a pelletingexperiment was carried out in each case with DL-methionine, MHA(methionine hydroxy analog) and calcium MHA. In addition, a controlexperiment was carried out by pelleting without addition of a methioninederivative (see table 5).

TABLE 5 Molecular Purity mass No. Methionine derivative (wt %) (monomer)Initial weight 1 No additive — —  0.00 g 2 DL-Methionine 99.0% 149.2150.61 g 3 MHA 88.0% 150.19 57.14 g 4 Calcium MHA (MHA-Ca) 93.3% 169.2260.77 g 5 DD/LL/DL/LD methionyl- 99.7% 140.20 47.13 g methionine (I)

All the diastereomers of methionylmethionine (I) remained stablethroughout the pelleting process and steam treatment (see table 6).

TABLE 6 Sample Feed mixture Feed pellets supplemented supplementedUnsupplemented with with Parameter feed mixture Met-Met (I) Met-Met (I)CP % 18.64 18.88 18.45 DM % 85.58 86.58 MET % 0.28 0.47 0.51 CYS % 0.320.32 0.30 MET + CYS % 0.59 0.79 0.81 LYS % 1.00 0.99 0.98 THR % 0.670.70 0.67 ARG % 1.16 1.19 1.17 ILE % 0.75 0.79 0.74 LEU % 1.54 1.60 1.51VAL % 0.88 0.90 0.85 HIS % 0.47 0.51 0.48 PHE % 0.91 0.92 0.88 GLY %0.78 0.81 0.77 SER % 0.89 0.94 0.90 ALA % 0.89 0.93 0.89 ASP % 1.74 1.751.70 GLU % 3.62 3.79 3.58 MET-MET Ex 0.156 0.153 (I) MET Ex 0.017 0.022LYS Ex 0.092 0.104 (Ex: soluble constituents)

In this case, the amino acid determination was based on EU method98/64/EC. After extraction of the free amino acids andmethionylmethionine (I), these were subsequently determined with the aidof an amino acid analyzer by post-column derivatization with ninhydrin(see table 6).

The leaching characteristics of the diastereomers of methionylmethionine(I) from the feed pellets was then investigated under water. In thiscase, the dissolving out of methionylmethionine under water as afunction of time, temperature, water composition (salt or fresh water)was determined. For this purpose, 20.0 g of the feed pellets were placedin a close-mesh sieve bag and completely immersed in 200 g of water inan Erlenmeyer flask. All the Erlenmeyer flask was subsequently agitatedcontinuously with a laboratory shaker at a constant temperature of 20°C. Then, at defined time intervals, a sample of water was removed ineach case and the content of the individual pairs of methionylmethioninediastereomers in the water was determined by HPLC (see table 7).

TABLE 7 DL/ LL/DD/ Time Methionine MHA MHA-Ca LL/DD-I LD-I LD/DL-I 04.0% 6.0% 8.6% 2.7% 0.6% 1.5% 5 12.0% 12.8% 16.5% 3.7% 0.7% 2.0% 1016.0% 20.8% 28.2% 6.5% 0.9% 3.2% 15 24.0% 28.8% 39.4% 7.7% 0.6% 3.6% 3039.9% 50.5% 61.7% 12.1% 0.6% 5.4% 60 59.9% 75.4% 82.4% 20.6% 1.7% 9.5%120 79.8% 94.1% 94.1% 27.4% 1.7% 12.3% 210 87.8% 99.9% 97.0% 35.9% 3.8%17.0%

For comparison, in each case the feed pellets supplemented withDL-methionine, MHA or calcium MHA were investigated under the sameconditions and thus their leaching characteristics under waterdetermined under the respective conditions (see FIG. 5 and table 7).

1-9. (canceled)
 10. A method for feeding an animal, comprising feedingthe animal a feed mixture comprising a nutrient selected from the groupconsisting of DL-methionyl-DL-methionine, a salt thereof and a mixtureof DL-methionyl-DL-methionine and a salt thereof, wherein the animal iskept in an aquaculture.
 11. The method as claimed in claim 10, where theanimal kept in an aquaculture is at least one animal selected from thegroup consisting of a salt water fish, a salt water crustacean, a freshwater fish and a fresh water crustacean.
 12. The method as claimed inclaim 10, wherein the animal kept in an aquaculture is an animalselected from the group consisting of a carp, a trout, a salmon, acatfish, a perch, a flatfish, a sturgeon, a tuna, an eel, a bream, acod, a shrimp, a hill and a prawn.
 13. The method as claimed in claim10, wherein the animal is selected from the group of animals consistingof a silver carp (Hypophthalmichthys molitrix), a grass carp(Ctenopharyngodon idella), a common carp (Cyprinus carpio), a bigheadcarp (Aristichthys nobilis), a carassius (Carassius carassius), a catla(Catla Catla), a Roho labeo (Labeo rohita), a Pacific salmon (Salmonsalar) an Atlantic salmon (Oncorhynchus kisutch), a rainbow trout(Oncorhynchus mykiss), an American catfish (Ictalurus punctatus), anAfrican catfish (Clarias gariepinus), a pangasius (Pangasius bocourtiand Pangasius hypothalamus), a Nile tilapia (Oreochromis niloticus), amilkfish (Chanos chanos), a cobia (Rachycentron canadum), a whitelegshrimp (Litopenaeus vannamei), a black tiger shrimp (Penaeus monodon)and a giant river prawn (Macrobrachium rosenbergii).
 14. The methodaccording to claim 10, wherein the nutrient is a salt ofDL-methionyl-DL-methionine and the salt comprises a cation selected fromthe group consisting of an alkali metal, an alkaline earth metal,ammonium, Cu²⁺, Zn²⁺, Co²⁺ and a mixture thereof.
 15. The methodaccording to claim 10, wherein a content of the nutrient in the animalfeed mixture is from 0.01 to 5% by weight.
 16. The method according toclaim 10, wherein the feed mixture further comprises: protein andcarbohydrate, obtained from a meal selected from the group consisting offish meal, soybean meal, corn meal, and mixtures thereof, and,optionally, a supplement selected from the group consisting of anessential amino acid, a protein, a peptide, a vitamin, a mineral, acarbohydrate, a fat, an oil and a mixture thereof.
 17. The methodaccording to claim 10, wherein the DL-methionyl-DL-methionine or saltthereof comprises one mixture selected from the group consisting of aDD/LL/LD/DL mixture, a DL/LD mixture, and a DD/LL mixture.
 18. Themethod according to claim 10, wherein the feed mixture further comprises0.01 to 20% by weight DL-methionine.
 19. The method according to claim10, wherein the DL-methionyl-DL-methionine is aDL/LD-methionylmethionine pair of enantiomers.
 20. The method accordingto claim 22, wherein the feed mixture further comprises 0.01 to 20% byweight DL-methionine.
 21. The method according to claim 10, wherein thefeed mixture is in the form of a pellet or an extrudate.
 22. The methodaccording to claim 10, wherein the nutrient is solely a DL/LDmethionylmethionine pair of enantiomers.
 23. The method according toclaim 10, wherein the nutrient does not comprise a DD enantiomer ofDL-methionyl-DL-methionine or a salt thereof